Silicon nitride can be produced according to several different techniques. One common method involves direct production from the elements silicon and nitrogen gas at temperatures between about 1150.degree. and 1300.degree. C. However, this process is strongly exothermic and the temperature must be carefully controlled and observed if a sufficiently high part of the alpha-phase, which is desired for the production of sintered products, shall be obtained. Local overheating occurs easily and leads to an increased formation of beta-phase which is not desirable. The starting materials are fairly expensive and there are high requirements on their purity and on an adjusted particle size.
For the production of silicon nitride from silicon dioxide, carbon and nitrogen, heat is supplied and the reaction is endothermic. The production method is considerably more simple and less sensitive than production from elementary silicon. The costs for the raw materials are also lower. However, the process results in a product having a higher content of impurities, for example of oxides and carbide, and particularly so at high temperatures and a high production rate.
Attempts have been made to solve the problems connected with the method based on silicon dioxide by using a considerable excess of carbon, which makes the formation of higher fractions of alpha-phase possible. However, problems will arise when the carbon excess has to be removed after the reaction. The common method of burning off the carbon excess results in a product having high contents of oxides. Other additions for influencing the process have also been suggested. However, these generally tend to add undesirable substances to the products and to increase the costs, among other things due to the fact that the additions must fulfill high demands on purity.